1. Technical Field:
The present invention relates to a piezoelectric sensor, an actuator whose distortion information is detected by this piezoelectric sensor, and a disk unit used for suppressing the distortion of the actuator according to the detected distortion information. More particularly, the present invention relates to a method for fixing the piezoelectric sensor to the actuator and checking the status of the electrical connection at the fixed portion.
2. Description of the Related Art:
Conventionally, electrodes have been formed on a thin plate-like piezoelectric element by plating those electrodes with such a conductive material as gold, nickel, tin, or the like. Generally, a pair of electrodes are formed with the same material in such a case. However, among those electrode materials, there have not been any materials that excel in both wettability required for soldering and adhesion required for bonding with an adhesive.
Basically, an adhesive is required to have the following characteristics: (1) The adhesive must flow smoothly. (2) The adhesive must flow into small spaces. (3) The adhesive must get electrode materials wet properly. (4) Finally, the adhesive must be set so as to form a tough layer and keep a high adhesion. More concretely, an adhesive must be fluid once. For example, it should preferably get the object electrode material wet with a low surface tension of liquid.
For example, gold has excellent wettability, but not excellent adhesion. On the contrary, nickel has excellent adhesion, but not excellent wettability. Consequently, in case of such the conventional technique, if a lead wire is to be soldered on one electrode of a piezoelectric sensor and the other electrode is to be bonded and fixed at a predetermined member thereof, either of the work properties must be sacrificed.
Under such the circumstances, the Applicant of the present invention has invented a method for bonding a piezoelectric sensor to a distortion detecting spot of an object member and examined the idea as follows. The piezoelectric sensor was provided with two electrodes, which are plated with nickel for excellent adhesion, and bonded at a distortion detecting spot of the object member with an adhesive as shown in FIGS. 9 and 10. (The configurations of the piezoelectric sensor shown in FIGS. 9 and 10 are unknown in the prior art; they are not included in any conventional techniques.)
The piezoelectric sensor 51 shown in FIG. 9 is structured so that the top and bottom surfaces of the piezoelectric element 51a are placed between nickel-plated electrodes. The electrode formed on the top surface is used as an output electrode 51b of the piezoelectric sensor and the electrode formed on the bottom surface is used to a ground electrode 51c thereof. The electrode 51c is used as a reference potential surface of the piezoelectric sensor. The surface of the ground electrode 51c is bonded to a distortion detecting spot of the object member 57 with an adhesive 56. In such a structure of the piezoelectric sensor, however, the adhesive surface of the ground electrode 51c is hidden completely. To avoid this, therefore, part of the ground electrode 51c is extended up to the top surface of the piezoelectric element and a lead 54 for taking out the reference potential is soldered at this portion.
In such a configuration, although the ground electrode 51c is grounded surely via the lead 54, the lead wire must be soldered to two electrodes of both output electrode 51b and ground electrode 51c. In addition, a special process is needed for extending part of the ground electrode 51c up to the top surface of the piezoelectric element. And, some more measures are further needed to improve both workability and cost of the piezoelectric sensor.
The bonding method shown in FIG. 10 is based on an idea that at least the predetermined distortion detecting spot of the object member 65 is formed with a conductive material and the predetermined spot is grounded. The piezoelectric sensor 61 is structured so that the top and bottom surfaces of the piezoelectric element 61a is placed between nickel-plated-electrodes. The electrode formed on the top surface is used as the output electrode 61b of the piezoelectric sensor 61 and the electrode formed on the bottom surface is used as the ground electrode 61c thereof. The electrode 61c is assumed as a potential surface of the piezoelectric sensor 61. The surface of the ground electrode 61c is then bonded and fixed to the distortion detecting spot, which is formed with a conductive material of the object member 65 with the use of a conductive adhesive 64.
Because the ground electrode 61c is grounded to the object member 65 directly in this case, the method has advantages that it is no need to solder any lead to the electrode and a general-purpose piezoelectric sensor is employable. In spite of this, the method still includes an anxiety; because the ground electrode 61c is possibly shorted with the output electrode 61b due to an adhesive extruded from the end of the bonding portion, the bonding must be done specially carefully. When in accordance with the bonding method example shown in FIG. 10, the higher the possibility of short-circuit becomes, the thinner the piezoelectric element becomes.
Under such the circumstances, it is an object of the present invention to provide a piezoelectric sensor that never sacrifices any of work properties, for example, even when a lead wire is to be soldered to one of a pair of electrodes of a piezoelectric sensor and the other electrode is to be bonded and fixed to a predetermined member with an adhesive.
It is another object of the present invention to make it efficient to bond the ground electrode of the piezoelectric sensor to the conductive portion of the object member with an adhesive so as to be fixed and connected electrically, thereby preventing a short-circuit that might occur between the two electrodes due to extruded and stuck adhesive.
It is further another object of the present invention to provide an easier method for testing whether or not the ground electrode is connected electrically to the conductive portion of the object member.
It is further another object of the present invention to provide a method for simplifying both wiring and assembling works for disposing the piezoelectric sensor to an actuator of a disk unit with the use of the above method.
The piezoelectric sensor, which is a first invention in this application, has a first electrode formed on a first surface of a thin plate-like piezoelectric element and a second-electrode formed on a second surface of the same piezoelectric element. The first electrode is formed with a conductive metallic material of more wetting properties than the second electrode and the second electrode is formed with a conductive metallic material of more adhesive properties than the first electrode.
For example, the first electrode may be plated with gold and the second electrode may be plated with nickel. According to the method for fixing the piezoelectric sensor, which is another invention in this application, in order to bond the piezoelectric sensor with an adhesive to an object member whose distortion is to be detected, a predetermined portion of the object member is formed with a conductive material and the second electrode of the piezoelectric sensor is bonded to the predetermined portion with a non-conductive adhesive, then pressed with a predetermined pressure so as to be hardened and connected.electrically. In this case, the adhesive may be an anaerobic photo-setting one.
The actuator, which is further another invention in this application, comprises an actuator arm held rotationally within a predetermined range and composed at least partially of a conductive material; a moving coil held at the actuator arm; a stator magnet cooperating with the moving coil to compose a voice coil motor and rotate the actuator arm; and the above piezoelectric sensor. The second electrode of the piezoelectric sensor is bonded to a predetermined conductive portion formed with the conductive material with a non-conductive adhesive, then pressed with a predetermined pressure so as to be hardened and connected electrically.
The disk unit, which is further another invention in this application, includes the above actuator and a control circuit for controlling a current flowing in the moving coil so as to suppress the distortion of the actuator arm according to a detected signal from the piezoelectric sensor. The predetermined conductive portion is grounded.
Furthermore, according to the method for checking connections, which is further another invention in this application, a test result of the electrical connection between the second electrode of the piezoelectric sensor bonded with the above fixing method and the predetermined portion of the object member is judged by the comparison of the capacities, capacity between both electrodes of the piezoelectric sensor before connection with capacity between the first electrode of the piezoelectric sensor and the predetermined portion of the object member after connection.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.